Step-by-step explanation:
Let F = The number of $5 bills and T = the number of $10 bills. She has 15 bills total, so:
F+T = 15
T = 15-F
She has $120 total:
$120 = ($5)*F + ($10)*T
$120 = $5*F + ($10)*(15-F) [Substitute 15-F in place of T]
120 = 5F + 150 - 10F
-30 = -5F
6 = F
Check:
T= 15-F = 15=6 = 9
$120 = ($5)*6 + ($10)*9
$120 = $30 + $90
$120 = $120
